1. Field of the Invention
The present invention generally relates to devices for interconnecting telecommunications lines, and more particularly to a method for preparing a ferrule used in a fiber optic connector.
2. Description of the Prior Art
In the last few years, optical fibers have replaced copper wire as the preferred medium for carrying telecommunications signals. As with copper wire, it is necessary to provide for the interconnection of optical fibers, during installation, repair or replacement of the fibers. There are generally two kinds of interconnection devices, splices and connectors. The term "splice" usually refers to a device which provides a permanent connection between a pair of optical fibers. The term "connector," in contrast, usually refers to a device which may be engaged and disengaged repeatedly, often with a different plug or receptacle. The present invention is directed to such a device, although the term "connector" should not be construed in a limiting sense since the present invention may inherently provide a permanent, as well as temporary connection.
There are two primary types of commercially available fiber optic connectors, ferrule connectors and biconic connectors. Ferrule connectors use a cylindrical alignment member, typically ceramic, having a central bore which receives a single optical fiber. Biconic connectors use a plug in the shape of a truncated cone. Both connectors usually combine a pair of plugs fitting into a common socket or receptacle to provide a completed connection. The prior art further recognizes that signal transmission across a connector may be improved by imparting an angled finish to the end face of the fibers. This construction results in reduced internal reflections of the signals at the end face. See, e.g., U.S. Pat. Nos. 5,062,682 and 5,066,094, European Patent No. 194,325, and Japanese Patent Application (Kokai) No. 1-121805.
The requirement for extremely low reflectivity connectors is becoming more stringent, particularly in high speed, single-mode fibers where laser sources are used. Cable television (CATV) operators presently require less than -50 dB reflectivity, and this standard will probably also be adopted by all telephony providers. Reflectivity performance must also be consistent, repeatable, and stable under environmental extremes. At the same time, insertion loss is required to be less than 0.5 dB. Early prior art focused on reducing reflections at the expense of higher insertion loss (attenuation), by providing an air gap between the angled connector fiber ends. See U.S. Pat. No. 4,695,126. In the ferrule construction of Japanese Patent Application (Kokai) No. 59-38707, the air gap is minimized by inclining only part of the ferrule end face, but this still degrades loss by at least 0.35 dB, and connection stability and repeatability is poor due to resonant Fresnel reflections.
The construction shown in U.S. Pat. No. 4,615,581 eliminates the air gap, but this construction utilizes a capillary which presents several new problems. First, the fiber must survive relatively sharp bends, which could cause fiber damage during insertion, and over the long term due to static fatigue. Secondly, the insert is difficult to reliably manufacture, and the resulting connector assembly is quite costly. Finally, slight end gapping may still occur if the ferrule end faces are not held extremely close to the 90.degree. interface. Another design (U.S. Pat. No. 4,978,193) attempts to overcome fiber gapping by applying the angular plane in an accruate factory process, and then field polishing only to the pre-angled surface, by using a hard ceramic ferrule material as a stop. This design requires rigid tolerances in keying the paired connectors together; if the key tolerances are not extremely tight, or change with axial or rotational disturbances, then an air gap will result, with all the corresponding drawbacks.
Loss values of less than 0.25 dB have been achieved in prior art angled connectors, typically by using ferrules whose ends have a spherical shape, to provide forcible physical contact (PC) between the interconnected fiber end faces, as illustrated in U.S. Pat. No. 5,140,660. This design also overcomes an earlier problem relating to forming an angled, spherical surface on a ferrule whose end is beveled or chamfered. The ferrules are chamfered in order to minimize the amount of grinding and polishing required to finish the ferrule end, and to facilitate proper alignment of the ferrules within the connector receptacle. As illustrated in FIG. 1, however, in forming an angled, spherical end face on such a chamfered ferrule 10, the apex 12 of the resulting spherical surface 14 does not coincide with the fiber axis 6, i.e., with the radial center of the ferrule (cf. FIG. 3 of the '660 patent). In this regard, the term "apex" as used herein refers to that point on spherical surface 14 whose tangent is generally parallel with the angle of inclination .theta. of spherical surface 14. The angle of inclination .theta. may further be generally defined with respect to the line formed between the diametrically opposed points A and B where spherical surface 14 intersects the chamfered surface 18. Those skilled in the art will appreciate, however, that surface 14 may not be perfectly spherical, and in fact is often aspherical near points A and B, although it very closely approximates a spherical surface near its center. Therefore, the tangent at apex 12 may not be perfectly parallel with the line between points A and B; similarly, the term "spherical" should not be construed in a limiting sense.
The non-coincidence of apex 12 and fiber axis 16 leads to a gap between the fiber ends (this effect is exaggerated in FIG. 1 due to the microscopic dimensions involved). The '660 patent suggests that this problem is unavoidable if the angled plane is adjacent to the circumferential chamfer, so it eliminates the problem by providing a smaller diameter tip on the ferrule which extends beyond the chamfer. The design of the '660 patent, however, still has several drawbacks. The design inherently requires a sharp leading edge of the angled end face, which can cause cutting, chipping or scraping of the various types of alignment sleeves and housings or receptacles, since the materials used in alignment components are soft compared with the zirconia and alumina ceramics typically used to fabricate the ferrules. This may create debris which can get lodged between the ferrule end faces and consequently degrade transmission. The stepped cylindrical shape of the '660 design also may not safely mate with the receptacles of existing or future electro-optic devices such as LED's (light-emitting diodes) and lasers, with the possibility of damaging such devices. It would, therefore, be desirable and advantageous to devise a method of imparting an angled finish to a ferrule end face which does not require the smaller diameter tip extending beyond the chamfer, and yet still centers the apex of the angled face very near the fiber axis in order to minimize any possible gap between the fiber ends.